A fractureability evaluation model based on brittleness, fracture toughness, and compressive strength and its application

Fracability serves as a critical indicator for assessing whether the production of unconventional oil and gas reservoirs can be increased by fracturing. Besides, it also represents a key parameter for evaluating shale oil wells. Currently, fracability evaluation is primarily conducted using the brittleness index based on the lithology or rock elasticity. However, due to the diversity of lithology and pore combinations, the elasticity and fracability of reservoirs may vary significantly even under identical lithological conditions. To address this issue, targeting the complex lithologies and diverse pores of shale oil reservoirs in the second member of the Kongdian Formation, Cangdong Sag, Bohai Bay Basin, this study proposes a reservoir fracability evaluation model that integrates the elastic-based brittleness index, compressive strength, and fault toughness. First, using the velocity and density data in array acoustic log data, elastic mechanical parameters, including the elastic modulus, Poisson's ratio, and Young's modulus, are calculated. Second, in combination with laboratory test results, the relationship between static and dynamic elastic moduli is established. Accordingly, elastic parameters, rock mechanics parameters, and the elastic-based brittleness index are determined. Third, for well blocks lacking shear wave information in conventional log data, a method for estimating the shear wave velocities of shales and sandstones is developed based on the Xu-White model, porosity, and contents of various minerals in shales. This model enables the calculation of parameters for the engineering sweet spots of reservoirs. Finally, a typical well in the study area is selected to calculate its fracability index (FI), followed by a grading evaluation. The results of this study indicate that FI values of < 0.45, 0.45~0.55, and ≥ 0.55 represent a low, moderate, and high fracability, respectively. The zones with low, moderate, and high fracability delineated in the study area agree well with the actual hydraulic fracturing results. This study provides a novel thought and technical approach for evaluating engineering sweet spots in shale oil exploration.